Covalent organic frameworks for direct photosynthesis of hydrogen peroxide from water, air and sunlight

Solar-driven photosynthesis is a sustainable process for the production of hydrogen peroxide, the efficiency of which is plagued by side reactions. Metal-free covalent organic frameworks (COFs) that can form suitable intermediates and inhibit side reactions show great promise to photo-synthesize H2O2. However, the insufficient formation and separation/transfer of photogenerated charges in such materials restricts the efficiency of H2O2 production. Herein, we provide a strategy for the design of donor-acceptor COFs to greatly boost H2O2 photosynthesis. We demonstrate that the optimal intramolecular polarity of COFs, achieved by using suitable amounts of phenyl groups as electron donors, can maximize the free charge generation, which leads to high H2O2 yield rates (605 μmol g−1 h−1) from water, oxygen and visible light without sacrificial agents. Combining in-situ characterization with computational calculations, we describe how the triazine N-sites with optimal N 2p states play a crucial role in H2O activation and selective oxidation into H2O2. We further experimentally demonstrate that H2O2 can be efficiently produced in tap, river or sea water with natural sunlight and air for water decontamination.

In this manuscript, the authors studied the relationship between photocatalytic performance and intramolecular polarity. They designed a series of COFs materials by using suitable amounts of phenyl groups as electron donors, that can maximize the free charge generation, leading to the record-high photocatalytic H2O2 synthesis (702 μmol g-1 h-1) from water, oxygen and visible light without requiring a sacrificial agent. They claim that the weak intramolecular polarity in D-A COFs constrains excitons dissociation, yet too strong intramolecular polarity inhibits excitons formation via a weakened π-conjugated effect. The concept is novel and interesting. I personally very like this idea. However, the three COFs have all been reported previously. More importantly, the structure analysis of COF-N31 looks problematic. The authors should address the questions below before publishing. A major revision is needed.
1.The main concern is the structure of COF-N31. The PXRD is unsatisfactory. I highly suggest the authors read this literature (J. Am. Chem. Soc. 2019, 141, 6152−6156) in which an exactly the same structure with COF-N31 was reported. The PXRD patterns look very different. The PXRD of the COF-N32 has a similar problem. The simulations and Pawley refinements of all three COFs should be done. The very related literature should be cited (J. Am. Chem. Soc. 2019, 141, 6152−6156;J. Am. Chem. Soc. 2017, 139, 13083−13091). Photocatalysis is a very complicated system that involves many factors. The crystallinity and porosity of the obtained COFs may also have significant influence on the catalytic activities. I would suggest improving the crystallinity and porosity of COF-N31 first before testing its photocatalytic activity.
2.The BET surface area of COF-N31 (81 m2/g) is much lower than COF-N32 (823 m2/g) and COF-N33 (677 m2/g). The standard of the materials is different for the three COFs. Although the author claimed that "The surface area and O2 absorption of COFs do not have close correlation with their H2O2 production." The reason and evidence should be provided.
3.TEM images of the three COFs should be provided.
4.The porous properties can not be seen from Scanning electron microscopy (SEM) images. 5.I would suggest to label the names of the three COFs in Figure 1a.

Reviewer #2 (Remarks to the Author):
The manuscript entitled "Engineering intramolecular polarity of covalent organic frameworks for boosting direct photosynthesis of hydrogen peroxide from water, air and sunlight" provide a strategy for the design of a novel donor-acceptor (D-A)-type COFs with various intramolecular polarity by using triazine-cored triamine with different amount of phenyl group (n = 0, 1, 2) as the precursors. The COF-N32 with moderate intramolecular polarity in the octupolar conjugated structure can maximize the free charge generation, leading to the record-high photocatalytic H2O2 synthesis from water, oxygen and visible light without requiring sacrificial agent. The COFs have been studied in detail experimentally and theoretically for their optoelectronic and photocatalytic properties, but some of the explanations were not clear enough. For this, I recommend this manuscript be published in Nature Communications with major revision, after considering the comments below. 1. X-ray diffraction (XRD) patterns of COF-N31 is not reasonable, because XRD peaks usually does not emerge after 10°. 2. As the VB level of COF-N32 is 1.88 V vs NHE, the overpotential for water oxidation is only <0.4 V, so the water oxidation to H2O2 seems to be difficult, and not well discussed in the current manuscript. 3. H2O2 production via two-electron water oxidation is an exciting work. But lack detail experimental evidences and not well discussed in the current manuscript. 4. Taking into account that the photo-generated holes could oxidize water to form hydroxyl radicals, could the formation of hydroxyl radicals affect the H2O2 production reaction? 5. In order to make the manuscript more general, the authors need to include these recent references. ACS Catal. 2022, 12, 12954-12963;Chemical Engineering Journal 454 (2023) 139929.

Reviewer #3 (Remarks to the Author):
The authors synthesized a class of D-A COFs with different intramolecular polarity by introducing suitable amounts of phenyl groups as electron donors for excitonic regulation to boost the direct photocatalytic H2O2 production from water, air, and sunlight without using sacrificial agent. The optimal COF-N32 can facilitate excitons formation and dissociation, leading to the record-high H2O2 yield (702 μmol g-1 h-1) with SCC of 0.31%. In addition, COF-N32 can also be verified the potential application feasibility with high photocatalytic stability. H2O2 photosynthesis is hot topic, and the photocatalytic performance reported in this manuscript is impressive. As a result, this reviewer would like to recommend the acceptance of this manuscript for publication in Nat. Commun. after clarifying the following issues.
1.In the paragraph describing "Photocatalytic H2O2 production by COFs", authors try to convince the readers of the best photocatalytic performance of COF-N32. They said those "H2O2 yield by COF-N32 reaches 702 μmol g-1 h-1 " and "which is much higher than those of recently reported photocatalysts in pure water under same measurement conditions (Figures 3b and S22, Table S3)." However, the given H2O2 yield was 3168 μmol g-1 h-1 in Figure 3b, which is taken form that in Table S3 and doesn't appear even once in the text. The amount of COF-N32 seems to lead to different yields, however there is no corresponding description in the text. Explanation should be provided for the graphic and text discrepancies.
2.The morphology of COFs is suggested to be further characterized by Transmission electron microscopy.
3.Rotating ring-disk electrode measurements are suggested to be carried out to explore the number of electron transferred during the catalytic reaction.
4.The standardization of various abbreviations and spelling should be carefully checked. For example, "COF-32" in lines 28 and 242 would be corrected to "COF-N32".

Reviewer #4 (Remarks to the Author):
In this paper, Tong et al. report a strategy to regulate the intramolecular polarity of COFs to enhance the generation and separation of photoinduced excitons, thus optimizing the performance of photocatalytic H2O2 generation by these COFs. They claimed that a COF with a moderate intramolecular polarity that contains a triazine core and two peripheral benzenes exhibited the greatest performance for H2O2 production. The generalizability of this COF photocatalyst was verified by varying the water source, assembling it into practical devices and reusing for several times, which gives satisfactory performance in all the cases.
The authors investigated the mechanism of H2O2 production by conducting quenching experiments, in-situ ESR analysis, in-situ FTIR analysis and DFT calculations, elucidating the simultaneous oxygen reduction and water oxidation pathways and revealing the triazine nitrogen and carbon as the active sites for water oxidation and O2 reduction, respectively. In summary, a comprehensive structure-property-function relationship was established in this paper, which provides a clear clue for the design of COFs via regulating their intramolecular polarity to boost photocatalytic performance. The whole manuscript is arranged logically and well-written. The figures and illustrations are appropriate for interpretation. The conclusion was well supported by the experimental data and analysis. Thus, I think this manuscript meets the criteria of Nature Communications and I recommend the publication of this manuscript after addressing the following concerns. 1, the author claimed the record-high H2O2 yield (702 μmol g-1 h-1) for a COF photocatalyst in the absence of a sacrificial reagent. From the viewpoint of charge conservation, theoretically, the electrons generated on COF which reduce oxygen should come from the oxidation of water. In order to exclude the self-oxidation of COF itself, H2O18 is suggested to use to verify the water oxidation process. The amount of 2 * 18O2 and H218O2 in the system should be equal to the H216O2 to meet the requirement of charge conservation. Otherwise, self-oxidation of COF cannot be excluded. 2, "The signal intensities of three COFs follow the order of COF-N31 > COF-N32 > COF-N33 ( Figure S5), which might result from the difference in intramolecular polarity among three COFs". As we know, the ESR signal comes from unpaired electrons in the materials. How do the authors draw a conclusion that different ESR signal intensities result from the different polarities in COFs? Is there any relationship between unpaired electrons and intramolecular polarity? 3, I doubt the relationship between water contact angles and polarity. Because measurement of water contact angles was in macroscale and the polarity was usually described in molecular scale. Also, the measurement of water contact angles will likely be influenced by many factors such as the preparation of samples, material morphology, temperatures, humidity, and so on. Alternatively, water adsorption analysis is suggested to study the polarity because the adsorption usually takes place in micro to mesoscale, which might be more convincing. 4, "Among three COFs, COF-N32 exhibits smaller average lifetime (86 ps) relative to COF-N31…" what kind of species do the lifetimes represent? photoexcited electrons? Holes? Or excitons? 5, The author performed the isopropanol (IPA) quenching experiment to trap the diffusing ·OH. However, isopropanol might act as a sacrificial agent which directly extracts holes. So is this appropriate to use IPA as the trapping agent of diffusing ·OH? 6, In Figure 2a, two transitions can be observed for COF-N33, which is different from COF-N31 and N33. How to explain this as the three COFs are anal

Reviewer #5 (Remarks to the Author):
In this work, the authors have synthesized and characterized a set of three covalent organic frameworks (COFs) that show a cost-effective approach to producing H2O2 from the air, water, and sunlight. In addition, they present a strategy for designing metal-free COFs with optimal properties to produce H2O2. The work is interesting, but there is still some additional work that needs to be done before its suitable for publication. As a result, I cannot recommend it for publication at the moment. My detailed comments are provided below: 1. The authors need to be explicit in the text about how they quantified the H2O2 that was produced. It was sort of frustrating to dig for this vital information. 2. The authors should check for the production of hydrogen and oxygen since the band gap suggests this material should be capable of oxidizing water to O2 and H2. 3. The authors should refrain from using SEM to qualify the porous nature of these materials. SEM would describe the morphology of the material. If the authors are adamant about using an imaging technique to report the pore size of the COFs, suggest they obtain highresolution TEM. Surprisingly, the author uses an imaging technique to qualify the pore size distribution of the material when the acceptable standard is to use BET analysis. Since the authors have already conducted BET measurements for these COFs to quantify the surface area of these COFs, they should report the pore-size distribution from the BET analysis. 4. The authors should obtain a better PXRD profile. I suggest using a Voltage of 30 and a Current of 20 and running the experiment for at least 40 minutes. Running PXRD with high power induces fluoresces in the material, resulting in poor peaks. 5. The authors should obtain simulated PXRDs of the COFs to determine the interlayer stacking of these materials. This information is crucial because the authors claim that these are novel COFs. Additionally, determining the interlayer stacking will also help in explaining their results.
The original COF-N31 was fabricated in the mixture of mesitylene, 1,4-dioxane and 3 M HAc solution by using the same methods as COF-N32 and COF-N33. Since its crystallinity was not very high, the XRD pattern of COF-N31 does not match well with the simulation. Similar observations have been reported in previous studies about COF-N31 fabrication by ball-milling (Chem. Commun., 2019,55, 167-170) or solvothermal methods (Chem. Commun., 2017,53, 9636-9639;J. Chromatogr. A, 2020J. Chromatogr. A, , 1619. To investigate the photocatalytic property of COF-N31-DMSO, UV-DRS ( Figure R1a) and steady-state PL spectra ( Figure R1b) have been conducted. COF-N31-DMSO also exhibit wide band gap (2.71 eV) and low PL intensity, suggesting that COF-N31-DMSO with high calculated intramolecular polarity exhibits low light absorption but high charge separation. This observation is consistent with that of the original COF-N31. Therefore, the conclusion about the influence of intramolecular polarity on the generation and separation of excitons is not significantly affected by the crystallinity of COF-N31.
Previous study shows that COF-N31-DMSO can be employed in organic reaction with good photo stability (J. Am. Chem. Soc., 2019, 141, 6152−6156). However, in present study, we find that the photo-stability is relatively low in pure water during H2O2 photosynthesis without the addition of sacrificial agent. Due to the high intramolecular polarity and high internal strain in COF-N31-DMSO, the intramolecular electric field intensity of C-N linkage may be high, leading to the high reactivity of self-decomposition in the C-N linkage (Nat Commun., 2018, 9, 2998Chem. Soc. Rev., 2020,49, 8469-8500). As a result, COF-N31-DMSO is relatively easy to self-decompose in water under light irradiation, leading to the relatively low photo-stability. Specifically, the H2O2 yield of COF-N31-DMSO decrease to only 77 μmol g -1 h -1 in the sixth reused cycle ( Figure R2a). Meanwhile, the gradual decrease in H2O2 yield is also observed within 12 h in water ( Figure  R2b).
In contrast, original COF-N31with relatively low crystallinity compared with COF-N31-DMSO exhibits high photo-stability ( Figure R2c) and can continuously produce H2O2 within 12 h under visible light irradiation in water ( Figure R2d). Therefore, the increase in crystallinity can not improve the overall photocatalytic H2O2 production by COFs in water without the addition of sacrificial agent. Instead, the intrinsic chemical structure would significantly dominate the photocatalytic capability, which has been demonstrated in many previous studies about conjugated polymers (e.g. Yan et al., Proc. Natl. Acad. Sci. U. S. A, 2022, 119, e2202913119;Ye et al., Proc. Natl. Acad. Sci. U. S. A, 2021, 118, e2103964118). Therefore, original COF-N31 fabricated by using the same methods as COF-N32 and COF-N33 is used in our following control experiments.

Synthesis of COF-N31-DMSO
"COF-N31-DMSO was fabricated by a solvothermal method according to the literature (J. Am. Chem. Soc., 2019, 141, 6152−6156). Specifically, 0.3 mmol of Tp and 0.3 mmol of melamine were added into a reactor, followed by the addition of 2 mL of dimethyl sulfoxide, 1 mL of N,N-dimethylacetamide (DMAc) and 0.3 mL of 6 M acetic acid. After ultrasonication and degassed by three consecutive freeze-pump-thaw cycles, the reactor was sealed under vacuum condition, which was then heated at 120 ℃ for 72 h. The precipitation was first rinsed by DMAc. Then, the product was solvent exchanged with DMAc, pure water and washed with acetone for three times. The product was dried at 120 ℃ under vacuum." Comment 2: The BET surface area of COF-N31 (81 m 2 g -1 ) is much lower than COF-N32 (823 m 2 g -1 ) and COF-N33 (677 m 2 g -1 ). The standard of the materials is different for the three COFs. Although the author claimed that "The surface area and O2 absorption of COFs do not have close correlation with their H2O2 production." The reason and evidence should be provided.

Response 2:
Thank you for pointing out our omission. After the careful consideration, the correlation of surface area and O2 absorption with H2O2 can not be totally excluded. Based on the results of BET surface area, TPD-O2 analysis and the production of 1,4-endoperoxide species and adsorbed ·O2 -, it can be only concluded that the electron transfer efficiency in oxygen reduction is more important than surficial properties during H2O2 production process. The corresponding texts has been rewritten in the caption of Figure S48. Figure S48. N2 adsorption-desorption isotherms of (a) COF-N31, (b) COF-N32 and (c) COF-N33 and the corresponding pore size distribution (insets). (d) O2-TPD curves of three COFs. The BET surface area of COF-N31, COF-N32 and COF-N33 is determined to be 81 m 2 /g, 823 m 2 /g and 677 m 2 /g, respectively. The three COFs shows narrow pore size of 1-2 nm, indicating their micropore structure. The intensities of O2-TPD signals follow the order of COF-N31 > COF-N32 > COF-N33. It should be noted that the COFs are thermally stable at the tested temperature (< 200℃, Figure S47), suggesting the signals in O2-TPD are not resulted from the decomposition of COFs. Therefore, the O2 adsorption is mainly relevant to the structure of COFs instead of surface area. Besides, COF-N31 with more O2 adsorption exhibits relatively low production capability of 1,4-endoperoxide species and adsorbed ·O2compared with that of COF-N32, further implying that the electron transfer efficiency in oxygen reduction is more important than surficial properties during H2O2 production process.
Comment 3: TEM images of the three COFs should be provided.

Comment 4:
The porous properties can not be seen from Scanning electron microscopy (SEM) images.

Response 4:
Thank you for your valuable comment. We agree the Reviewer that SEM could only show morphology, instead of porous structure. The BET measurement has been added to identify the porous properties of COF samples. The three COFs shows narrow pore size of 1-2 nm, indicating their micropore structure. The corresponding text has been rewritten in the revised manuscript (lines 72-74 and Figure S48). "Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images show that the three COFs are tiny granular particles with diameter of 2~3 m, which are assembled by numerous nanorods (Figures S3 and S4)." Comment 5: I would suggest to label the names of the three COFs in Figure 1a.
Response 5: Thank you for your valuable comment. Following your excellent suggestion, we have added the names of three COFs in Figure 1a.

Response to Reviewer 2：
Overall comments: The manuscript entitled "Engineering intramolecular polarity of covalent organic frameworks for boosting direct photosynthesis of hydrogen peroxide from water, air and sunlight" provide a strategy for the design of a novel donor-acceptor (D-A)-type COFs with various intramolecular polarity by using triazine-cored triamine with different amount of phenyl group (n = 0, 1, 2) as the precursors. The COF-N32 with moderate intramolecular polarity in the octupolar conjugated structure can maximize the free charge generation, leading to the record-high photocatalytic H2O2 synthesis from water, oxygen and visible light without requiring sacrificial agent. The COFs have been studied in detail experimentally and theoretically for their optoelectronic and photocatalytic properties, but some of the explanations were not clear enough. For this, I recommend this manuscript be published in Nature Communications with major revision, after considering the comments below.
Response: Thanks so much for your great efforts in reviewing our revised manuscript. We sincerely appreciate your valuable comments and suggestions. We have revised the manuscript according to your valuable suggestions as well as those from other Reviewers. We believe that the revised manuscript has been substantially strengthened. We are looking forward to your continuous support for our revised manuscript.
Comment 1: X-ray diffraction (XRD) patterns of COF-N31 is not reasonable, because XRD peaks usually does not emerge after 10°.

Comment 2:
As the VB level of COF-N32 is 1.88 V vs NHE, the overpotential for water oxidation is only <0.4 V, so the water oxidation to H2O2 seems to be difficult, and not well discussed in the current manuscript.
Response 2: Thank you for your valuable comment. According to the 18 O isotopic experiment, H2 18 O2 was generated by using H2 18 O and 16 O2 in COF-N32 system, confirming the two-electron water oxidation by COF-N32 under visible light irradiation. According to the DFT calculation, the energy barrier (Figure 5d) of water oxidation is relatively high compared with oxygen reduction (Figure 5e). Therefore, water oxidation is the rate-determining step, rather than the O2 reduction. The direct conversion of *OH to * + H2O2 is a quick interfacial reaction, which is not relevant to the diffusion, leading to the low requirement of overpotentials in COF-N32 photocatalyst. Similar observations have been also reported in the previous studies about organic photocatalysts for two-electron water oxidation (Adv. Mater., 2020, 32, e1904433;Adv. Mater., 2022, 34, 2107480). The corresponding explanations have been added into the revised manuscript (Please see lines 240-241). "This allows the occurrence of water oxidation with relatively low overpotential of VB compared with E(H2O2/H2O) (1.77 V vs. NHE) 49 ." Comment 3: H2O2 production via two-electron water oxidation is an exciting work. But lack detail experimental evidences and not well discussed in the current manuscript.
Response 3: Thank you for your valuable comment. As referred in the Response 2, 18 O isotopic experiment was conducted to confirm the two-electron water oxidation. By using H2 18 O and 16 O2 as precursors, H2O2 was generated by COF-N32 under visible light irradiation. After purging N2 to remove excessive 16 O2 and air, catalase was introduced into the sealed reactors to decompose the generated H2O2. The gas was extracted and determined by GC-MS. The result showed that 16 O2 and 18 O2 were generally equivalent (Figure S44), indicating that two-electron water oxidation was conducted during H2O2 production in COF-N32 system. In addition, oxygen evolution reaction was also performed. By using NaBrO3 as an electron scavenger in Ar atmosphere, negligible amount of O2 was generated by COF-N32 after 3 h of visible light irradiation ( Figure  S43). The observation further confirms that the water oxidation in COF-N32 system is a two-electron pathway, rather than four-electron pathway.
The corresponding explanations have been added into the revised manuscript (Please see line 242-247). "… while the negligible O2 generation in oxygen evolution experiment excluded four-electron water oxidation ( Figure S43). The isotopic experiment was further conducted by using H2 18 O and 16 O2 as precursors in a sealed reactor. The result shows that the amount of 16 O2 and 18 O2 generated from the decomposition of H2O2 is generally equivalent (Figure S44), which further confirms the presence of two-electron water oxidation as well as the charge conservation with oxygen reduction during the H2O2 production process." Figure S43. The oxygen evolution by COF-N32 in the presence of NaBrO3 under Ar atmosphere. Conditions: λ>420 nm (298K; xenon lamp, light intensity:100 mW·cm -2 ), ultrapure water (50 mL). Figure S44. Isotopic experiment by using H2 18 O as water source during H2O2 photosynthesis Comment 4: Taking into account that the photo-generated holes could oxidize water to form hydroxyl radicals, could the formation of hydroxyl radicals affect the H2O2 production reaction?
Response 4: Thank you for your valuable comment. Photo-generated holes in some photocatalysts (e.g. TiO2) with low valence band potentials could oxidize water to free hydroxyl radicals with E(·OH/H2O) = 2.72 V vs NHE. However, in this study, the valence band potential of COF-N32 is determined to be 1.88 V vs NHE, which is much smaller than E(·OH/H2O). Thus, the oxidation of H2O to hydroxyl radicals is not available in thermodynamics. Consistently, no obvious signal for ·OH can be detected by using ESR analysis ( Figure S37). Moreover, the introduction of TBA (the scavenger of diffusing ·OH) does not obviously affect the H2O2 photosynthesis (Figure 5a), which also confirms the insignificant role of diffusing ·OH radicals. The corresponding explanations have been added into the revised manuscript (Please see lines 227-229). "Meanwhile, the introduction of tertiary butanol (TBA) has negligible effect on H2O2 yield by COF-N32 (p>0.1), indicating that diffusing ·OH does not have contribution to the photocatalytic process of H2O2 production."  Comment 5: In order to make the manuscript more general, the authors need to include these recent references. ACS Catal. 2022, 12, 12954-12963;Chemical Engineering Journal 454 (2023) 139929.
Response 5: Thank you for your valuable suggestion. We have included the references in the revised manuscript.

Response to Reviewer 3：
Overall comments: The authors synthesized a class of D-A COFs with different intramolecular polarity by introducing suitable amounts of phenyl groups as electron donors for excitonic regulation to boost the direct photocatalytic H2O2 production from water, air, and sunlight without using sacrificial agent. The optimal COF-N32 can facilitate excitons formation and dissociation, leading to the record-high H2O2 yield (702 μmol g -1 h -1 ) with SCC of 0.31%. In addition, COF-N32 can also be verified the potential application feasibility with high photocatalytic stability. H2O2 photosynthesis is hot topic, and the photocatalytic performance reported in this manuscript is impressive. As a result, this reviewer would like to recommend the acceptance of this manuscript for publication in Nat. Commun. after clarifying the following issues.
Response: Thanks so much for your great efforts in reviewing our revised manuscript. We sincerely appreciate your valuable comments and suggestions. We have revised the manuscript according to your valuable suggestions as well as those from other Reviewers. We believe that the revised manuscript has been substantially strengthened. We are looking forward to your continuous support for our revised manuscript.
Comment 1: In the paragraph describing "Photocatalytic H2O2 production by COFs", authors try to convince the readers of the best photocatalytic performance of COF-N32. They said those "H2O2 yield by COF-N32 reaches 702 μmol g -1 h -1 " and "which is much higher than those of recently reported photocatalysts in pure water under same measurement conditions (Figures 3b and S22, Table S3)." However, the given H2O2 yield was 3168 μmol g -1 h -1 in Figure 3b, which is taken form that in Table S3 and doesn't appear even once in the text. The amount of COF-N32 seems to lead to different yields, however there is no corresponding description in the text. Explanation should be provided for the graphic and text discrepancies.
Response: Thank you for your careful reading and value comment. The Reviewer is correct that lowering the dosage of photocatalysts would improve the mass activity. The H2O2 yields by COF-N32 is higher than other reported photocatalysts under the same dosage. Following your excellent suggestion, we have added the corresponding text in the main manuscript (Please see lines 170-172 and Figure S24). "Moreover, COF-N32 can yield over 3.17 mmol g -1 h -1 with the addition of 1 mg COF-N32 in 50 mL ultrapure water ( Figure S24)…" Figure S24. H2O2 yield by COF-N32 with different dosage in 50 mL O2-saturated ultrapure water. Conditions: λ>420 nm (298K; xenon lamp, light intensity:100 mW·cm -2 ), ultrapure water (50 mL). COF-N32 can yield over 3168 μmol g -1 h -1 and 1612 μmol g -1 h -1 with the addition of 1 and 5 mg COF-N32 in 50 mL ultrapure water, respectively, which are much higher than those of recently reported photocatalysts in pure water under the same measurement conditions (Table S3).

Comment 2:
The morphology of COFs is suggested to be further characterized by Transmission electron microscopy.
Response 2: Thank you for your valuable comment. Following your excellent suggestion, we have added the TEM images in the revised Supporting Information (Please see Figure S4). The corresponding texts have been added into the revised manuscript (Please see Lines 72-74). "Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images show that the three COFs are tiny granular particles with diameter of 2~3 m, which are assembled by numerous nanorods (Figures S3 and S4)." In COF-N33 with high crystallinity, the diffraction fringes of (100) and (002) planes can be found in Figure S4e and S4f, respectively.

Comment 3:
Rotating ring-disk electrode measurements are suggested to be carried out to explore the number of electrons transferred during the catalytic reaction.
Response 3: Thank you for your valuable comment. Following the Reviewer's excellent suggestion, rotating ring-disk electrode analysis is used to determine the number of electrons transfer from COF-N32 to O2. The result shows that the n was 2.17 for COF-N32, suggesting that the oxygen reduction reaction was apparent 2electron reaction. The corresponding texts have added in the revised manuscript (Please see lines 214-217). "Rotating ring-disk electrode (RRDE) analysis shows that the number of electrons transferred from COF-N32 to O2 is estimated to be 2.17 (Figure S37), indicating that O2 is generally reduced to generate H2O2 via the apparent 2-electron reaction. The intermediates during oxygen reduction were further investigated by the trapping experiments." Figure S37. (a) RRDE curves over COF-N32-coated electrodes measured at 1600 rpm in O2-saturated electrolyte using the ring current (top) and the disk current (bottom). (b) The average number of the transferred electrons (n) at different potentials calculated from RRDE data.

Electrochemical measurement
"Rotating ring-disk electrode (RRDE) analysis was conducted in a three-electrode cell by using Pt foil as a counter electrode and using Ag/AgCl as a reference electrode, respectively. The RRDE was consist of a glassy carbon disk and Pt ring. Before the experiment, COF-N32 was dropped onto the glassy carbon disk and then dried."

Comment 4:
The standardization of various abbreviations and spelling should be carefully checked. For example, "COF-32" in lines 28 and 242 would be corrected to "COF-N32".
Response 4: Thank you for your pointing out our omission. Following your valuable comment, we have corrected the mistakes in the revised manuscript.

Response to Reviewer 4：
Overall comments: In this paper, Tong et al. report a strategy to regulate the intramolecular polarity of COFs to enhance the generation and separation of photoinduced excitons, thus optimizing the performance of photocatalytic H2O2 generation by these COFs. They claimed that a COF with a moderate intramolecular polarity that contains a triazine core and two peripheral benzenes exhibited the greatest performance for H2O2 production. The generalizability of this COF photocatalyst was verified by varying the water source, assembling it into practical devices and reusing for several times, which gives satisfactory performance in all the cases. The authors investigated the mechanism of H2O2 production by conducting quenching experiments, in-situ ESR analysis, in-situ FTIR analysis and DFT calculations, elucidating the simultaneous oxygen reduction and water oxidation pathways and revealing the triazine nitrogen and carbon as the active sites for water oxidation and O2 reduction, respectively. In summary, a comprehensive structure-property-function relationship was established in this paper, which provides a clear clue for the design of COFs via regulating their intramolecular polarity to boost photocatalytic performance. The whole manuscript is arranged logically and well-written. The figures and illustrations are appropriate for interpretation. The conclusion was well supported by the experimental data and analysis. Thus, I think this manuscript meets the criteria of Nature Communications and I recommend the publication of this manuscript after addressing the following concerns.
Response: Thanks so much for your great efforts in reviewing our revised manuscript. We sincerely appreciate your valuable comments and suggestions. We have revised the manuscript according to your valuable suggestions as well as those from other Reviewers. We believe that the revised manuscript has been substantially strengthened. We are looking forward to your continuous support for our revised manuscript.

Comment 1:
The author claimed the record-high H2O2 yield (702 μmol g -1 h -1 ) for a COF photocatalyst in the absence of a sacrificial reagent. From the viewpoint of charge conservation, theoretically, the electrons generated on COF which reduce oxygen should come from the oxidation of water. In order to exclude the selfoxidation of COF itself, H2O 18 is suggested to use to verify the water oxidation process. The amount of 2 * 18 O2 and H2 18 O2 in the system should be equal to the H2 16 O2 to meet the requirement of charge conservation. Otherwise, self-oxidation of COF cannot be excluded.
Response 1: Thank you for your valuable comment. Following your excellent suggestion, 18 O isotopic experiment was performed to confirm the charge conservation between water oxidation and oxygen reduction. Typically, the H2O2 photosynthesis by COF-N32 was conducted in H2 18 O under 16 O2 atmosphere. After the visible irradiation, N2 was purged into the reaction system to remove excessive 16 O2 and air in the sealed reactor, followed by the injection of catalase to decompose the produced H2O2 into O2. The gas was extracted and injected into GC-MS for analysis. The result showed that 16 O2 and 18 O2 were generally equivalent ( Figure  S44). The observation indicates that the amount of H2 16 O2 is equal to the amount of H2 18 O2, which confirms the charge conservation between water oxidation and oxygen reduction. The corresponding explanations have been added into the revised manuscript (Please see lines 243-248 in main manuscript and lines 163-168 in SI). "…while the negligible O2 generation in oxygen evolution experiment excluded four-electron water oxidation ( Figure S43). The isotopic experiment was further conducted by using H2 18 O and 16 O2 as precursors in a sealed reactor. The result shows that the amount of 16 O2 and 18 O2 generated from the decomposition of H2O2 is generally equivalent (Figure S44), which further confirms the presence of two-electron water oxidation as well as the charge conservation with oxygen reduction during the H2O2 production process." Figure S44. Isotopic experiment by using H2 18 O as water source during H2O2 photosynthesis 18 O isotopic experiment " 18 O isotopic experiment on COF-N32 was performed by using H2 18 O with saturated 16 O2 in a sealed reactor, which was irradiated by Xenon lamp (>420 nm). After H2O2 photosynthesis, the reaction suspension was purged by N2 to remove 16 O2 in the reactor. Subsequently, the photo-generated H2O2 was decomposed into O2 by adding catalase. The evolved O2 gas was analyzed by a gas chromatography-mass spectrometry (GC-MS, Agilent 7890B-5977B). " Comment 2: "The signal intensities of three COFs follow the order of COF-N31 > COF-N32 > COF-N33 ( Figure S5), which might result from the difference in intramolecular polarity among three COFs". As we know, the ESR signal comes from unpaired electrons in the materials. How do the authors draw a conclusion that different ESR signal intensities result from the different polarities in COFs? Is there any relationship between unpaired electrons and intramolecular polarity?
Response 2: Thank you for pointing out our omission. The solid-state ESR signal should be attributed to partial charge separation and the production of unpair electrons, which was driven by the electron push-pull effect of donor-acceptor structure (Liu et al., ACS Catal., 2022, 12, 9494-9502;Yan et al., Proc. Natl. Acad. Sci. U. S. A, 2022, 119, e2202913119). To avoid confusion, the corresponding text has been rewritten in the revised manuscript (Please see lines 88-89). "…indicating that different amounts of unpaired electrons exist in three D-A COFs under dark condition 30 ." Comment 3: I doubt the relationship between water contact angles and polarity. Because measurement of water contact angles was in macroscale and the polarity was usually described in molecular scale. Also, the measurement of water contact angles will likely be influenced by many factors such as the preparation of samples, material morphology, temperatures, humidity, and so on. Alternatively, water adsorption analysis is suggested to study the polarity because the adsorption usually takes place in micro to mesoscale, which might be more convincing.
Response 3: Thank you for your valuable comment. Following the Reviewer's excellent suggestion, we have also conducted the water adsorption analysis. The result shows that the unit water adsorption capacity also follows the order of COF-N31 > COF-N32 > COF-N33, which is consistent with the order of intramolecular polarity. The corresponding texts have been added into the revised manuscript (Lines 106-108). "Meanwhile, the unit water adsorption capacity is also consistent with the order of COF-N31 > COF-N32 > COF-N33 ( Figure S10)." Figure S10. The water adsorption isotherms of COF-N31, COF-N32 and COF-N33, which is normalized by BET surface area. However, isopropanol might act as a sacrificial agent which directly extracts holes. So is this appropriate to use IPA as the trapping agent of diffusing ·OH?
Response 5: Thank you for your valuable comment. We agree with the Reviewer that high concentration of IPA would react with holes and might not be appropriate to quench diffusing ·OH. According to previous studies, tertiary butanol (TBA) can be the scavenger of diffusing ·OH with the high kinetic constants of 3.8-7.6 × 10 8 M −1 s −1 . Thus, we also perform the quenching experiment by using TBA. The result shows that no obvious influence of TBA on the H2O2 photosynthesis by COF-N32 (p>0.1), indicating that diffusing ·OH plays negligible role in the photocatalytic H2O2 production process by COF-N32. The corresponding texts have been rewritten in the revised manuscript (Please see lines 227-229). "Meanwhile, the introduction of tertiary butanol (TBA) has negligible effect on H2O2 yield by COF-N32 (p>0.1), indicating that diffusing ·OH does not contribute to the photocatalytic process of H2O2 production."  Comment 6: In Figure 2a, two transitions can be observed for COF-N33, which is different from COF-N31 and N32. How to explain this as the three COFs are anal Response 6: Thank you for your valuable comment. All three COFs exhibit n-π* transition (particularly in triazine N atoms), while COF-N33 with more benzene units also contains more obvious signals of π-π* transition. Similar observation has been reported in the previous study about anthracene-containing COFs (Angew. Chem. Int. Ed., 2015, 54, 8704-8707). The corresponding texts have been added into the revised main manuscript (Lines 129-130). "Tauc plot based on UV-vis diffused reflectance spectra (DRS) reveals that all three COFs exhibit n-π* transition in N atoms 39 , while COF-N33 with more benzene units also contains more obvious signals of π-π* transition 40 ." Overall comments: In this work, the authors have synthesized and characterized a set of three covalent organic frameworks (COFs) that show a cost-effective approach to producing H2O2 from the air, water, and sunlight. In addition, they present a strategy for designing metal-free COFs with optimal properties to produce H2O2. The work is interesting, but there is still some additional work that needs to be done before its suitable for publication. As a result, I cannot recommend it for publication at the moment. My detailed comments are provided below: Response: Thanks so much for your great efforts in reviewing our revised manuscript. We sincerely appreciate your valuable comments and suggestions. We have revised the manuscript according to your valuable suggestions as well as those from other Reviewers. We believe that the revised manuscript has been substantially strengthened. We are looking forward to your continuous support for our revised manuscript.

Comment 1:
The authors need to be explicit in the text about how they quantified the H2O2 that was produced. It was sort of frustrating to dig for this vital information.
Response 1: Thank you for your pointing out our omission. We have added the corresponding texts in the revised Supporting Information (Please see lines 70-76 in SI).
The measurement of H2O2 "The production of H2O2 was measured by an iodometry method according to the literature (Energy & Environmental Science, 2018, 11, 2581-2589. Specifically, 1 mL of samples was added to the mixture of 1 mL of 0.4 M KI and 1 mL 0.1 M potassium hydrogen phthalate (C8H5KO4), which was kept for 1 h. Under acidic conditions, H2O2 can react with Ito generate triiodide anions (I3 -), which exhibited absorption at 350 nm. Thus, the absorbance at 350 nm by using UV-vis spectroscopy can measure the amount of I3 -, which can further determine the amount of H2O2 produced in each sample." Comment 2: The authors should check the production of hydrogen and oxygen since the band gap suggests this material should be capable of oxidizing water to O2 and H2.
Response 2: Thank you for your valuable comment. Following the Reviewer's valuable comment, we have tested the production of H2 during the H2O2 photosynthesis in a sealed reactor. However, no production of H2 could be observed (Figure R3), indicating that H + preferred to react with O2/·O2in COF-N32 system. In addition, we used NaBrO3 as the electron scavenger to investigate the oxygen evolution reaction on COF-N32 ( Figure S43). Yet, O2 could be hardly produced via the four-electron pathway. Instead, two-electron water oxidation was conducted on COF-N32 to generate H2O2. Figure R3. The hydrogen evolution by COF-N32. Conditions: λ>420 nm (298K; xenon lamp, light intensity:100 mW·cm-2), ultrapure water (50 mL). Figure S43. The oxygen evolution by COF-N32 in the presence of NaBrO3 under Ar atmosphere. Conditions: λ>420 nm (298K; xenon lamp, light intensity:100 mW·cm -2 ), ultrapure water (50 mL).

Comment 3:
The authors should refrain from using SEM to qualify the porous nature of these materials. SEM would describe the morphology of the material. If the authors are adamant about using an imaging technique to report the pore size of the COFs, suggest they obtain high-resolution TEM. Surprisingly, the author uses an imaging technique to qualify the pore size distribution of the material when the acceptable standard is to use BET analysis. Since the authors have already conducted BET measurements for these COFs to quantify the surface area of these COFs, they should report the pore-size distribution from the BET analysis.
Response 3: Thank you for your valuable comment. We agree the Reviewer that SEM could only show morphology, instead of porous structure. Following the Reviewer's excellent suggestion, we have added highresolution TEM in Figure S4. The pore size distribution is also provided in Figure S48, which shows the micropore structure of three COFs. The corresponding text has been added in the revised manuscript (Please see line 71-73 and Figure S48). "Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images show that the three COFs are tiny granular particles with diameter of 2~3 m, which are assembled by numerous nanorods (Figures S3 and S4)." Figure S4. TEM images of (a, b) COF-N31, (c, d) COF-N32 and (e, f) COF-N33. The diffraction fringes can not be observed in COF-N31 and COF-N32 due to the relatively low crystallinity of COFs compared with inorganic semiconductors (J. Am. Chem. Soc., 2017, 139, 13083−13091;J. Am. Chem. Soc., 2019, 141, 6152−6156;Chem. Commun., 2019,55, 167-170). In COF-N33 with high crystallinity, the diffraction fringes of (100) and (002) planes can be found in Figure S4e and S4f, respectively. Figure S48. N2 adsorption-desorption isotherms of (a) COF-N31, (b) COF-N32 and (c) COF-N33 and the corresponding pore size distribution (insets). (d) O2-TPD curves of three COFs. The BET surface area of COF-N31, COF-N32 and COF-N33 is determined to be 81 m 2 /g, 823 m 2 /g and 677 m 2 /g, respectively. The three COFs shows narrow pore size of 1-2 nm, indicating their micropore structure. The intensities of O2-TPD signals follow the order of COF-N31 > COF-N32 > COF-N33. It should be noted that the COFs are thermally stable at the tested temperature (< 200℃, Figure S47), suggesting the signals in O2-TPD are not resulted from the decomposition of COFs. Therefore, the O2 adsorption is mainly relevant to the structure of COFs instead of surface area. Besides, COF-N31 with more O2 adsorption exhibits relatively low production capability of 1,4-endoperoxide species and adsorbed ·O2compared with that of COF-N32, further implying that the electron transfer efficiency in oxygen reduction is more important than surficial properties during H2O2 production process.

Comment 4:
The authors should obtain a better PXRD profile. I suggest using a Voltage of 30 and a Current of 20 and running the experiment for at least 40 minutes. Running PXRD with high power induces fluoresces in the material, resulting in poor peaks.
Comment 5: The authors should obtain simulated PXRDs of the COFs to determine the interlayer stacking of these materials. This information is crucial because the authors claim that these are novel COFs. Additionally, determining the interlayer stacking will also help in explaining their results.
Moreover, the H2O2 production by both COF-N31 and COF-N33 during reused cycles were also investigated. The crystalline structure of both COF-N31 and COF-N33 after use was also characterized ( Figure S26). We found that in pure water, COF-N31 with strong intramolecular polarity has low photo-stability of COF-N31 during the reaction duration ( Figure S26-28), while COF-N32 and COF-N33 with relatively weak intramolecular polarity especially COF-N32 contain excellent photo-stability under visible light irradiation and can be consecutively reused for the photo-generation of H2O2. The corresponding texts have been incorporated in the 2 nd revised manuscript: "The stable H2O2 yield during the 5 reused cycles and no obvious change of crystalline structure after use are also achieved for COF-N33 (Figures S26a and S26b). In contrast, the H2O2 yield by COF-N31 dramatically decreases with increasing reused cycles (Figure S26c), indicating the relatively low photo-stability of COF-N31 during the reaction duration. The decreased crystallinity of COF-N31 after photocatalytic reaction ( Figure S26d)  The figures with updated results of the newly prepared COF-N31 in the 2 nd revised manuscript are listed as follows:       The diffraction fringes can not be observed in COF-N31 and COF-N32 due to the relatively low crystallinity of COFs compared with inorganic semiconductors S1, S4-5 . In COF-N33 with high crystallinity, the diffraction fringes of (100) and (002) planes can be found in Figure S4e and S4f, respectively.                  Figure S41. N2 adsorption-desorption isotherms of (a) COF-N31, (b) COF-N32 and (c) COF-N33 and the corresponding pore size distribution (insets). (d) O2-TPD curves of three COFs. The BET surface area of COF-N31, COF-N32 and COF-N33 is determined to be 75 m 2 /g, 823 m 2 /g and 677 m 2 /g, respectively. The three COFs shows narrow pore size of 1-2 nm, indicating their micropore structure. The intensities of O2-TPD signals follow the order of COF-N31 > COF-N32 > COF-N33. It should be noted that the COFs are thermally stable at the tested temperature (< 200℃, Figure S40), suggesting the signals in O2-TPD are not resulted from the decomposition of COFs. Therefore, the O2 adsorption is mainly relevant to the structure of COFs instead of surface area. Besides, COF-N31 with more O2 adsorption exhibits relatively low production capability of ·O2and H2O2 compared with that of COF-N32, further implying that the electron transfer efficiency in oxygen reduction is more important than surficial properties during H2O2 production process.